A conventional rail vehicle braking system includes a pneumatic pipe referred to as the "main brake-pipe" which runs along the train. The compressed air pressure level in the pipe determines the reference level at which the brakes are applied (or released). For reasons of safety, when the main brake-pipe is at atmospheric pressure, the braking reference is at its maximum. Conversely, when the pressure is nominal in the main brake-pipe, no braking is required. The braking reference is therefore given by the pressure level in the main brake-pipe, and is transformed by a distributor into a pressure value (or a suction value) for controlling a flow-rate relay whose function is to fill (or to empty) the brake cylinders on the basis of the accumulated pressure in a compressed air store referred to as the auxiliary tank. Between the flow-rate relay and the brake cylinder, there is generally a solenoid valve referred to as an "anti-skid exhaust relay" which is controlled by an electronic circuit whose function is to control the slip of the axles, and to prevent locking by forcing the brake cylinders to empty into the atmosphere.
In general, the brake cylinders include slack adjusters to compensate for wear due to friction. In principle, the adjusters are integrated in the bogies and they actuate a mechanical linkage which amplifies the force with which the friction pads are applied against the energy-dissipating members constituted by the wheels and/or the disks. The pneumatic feed circuit for the brake cylinders on the bogie is connected to the circuit on the vehicle body via a hose pipe.
Depending on the complexity of the systems that are used conventionally, the relays may or may not have pressure amplifying ratios, and they may take into account external data so as to modulate the pressure in the brake cylinder as a function of said data.
Much research is currently being done on braking trains by means of disks and of pads made of material based on carbon. This type of braking is already used in aviation, and on Formula 1 cars. Its main advantage lies in its capacity to absorb very considerable amounts of energy. It is thought that this type of braking could be used advantageously in braking very high speed trains whose current braking systems are close to their limits. It would also make it possible to increase urban and suburban traffic densities (subway trains, suburban trains, and railcars, etc.). Furthermore, the light weight of this type of system would enable non-suspended masses to be reduced significantly.
However, adapting this braking system to rail vehicles is not easy, as demonstrated by the article written by Jacques Raison and entitled "Les materiaux de freinage" (Braking materials), published in the Revue Generale des Chemins de Fer, July-August 1991. Such carbon-based materials normally work at high temperatures (of about 1,000.degree. C.) which, due to their closeness to the heat-sensitive surrounding portions and to heat conduction, give rise to high temperatures in said heat-sensitive surrounding portions (e.g. bearings). Another drawback results from the fact that the coefficient of friction of the materials varies as a function of speed, specific pressure, and humidity. This phenomenon is not a very considerable drawback in aviation or in Formula 1, since the aircraft pilot or the racing driver modulates the force he applies on the brakes as a function of what he feels. The same does not apply to rail vehicle braking, where the train driver does not directly feel the consequences of the braking force.